Toroidal fluid swivels are known in the art for transfer of high-pressure fluids across a rotary interface between an incoming fluid line and an outgoing product piping. Applications for such a swivel include for example offshore oil and gas explorations where high-pressure flows of oil and/or gas are transferred from a (deep-sea) offshore well to a floating vessel such as a Floating Production Storage and Offloading (FPSO) vessel. Typically such a floating vessel is equipped with a turret mooring system that can couple a mooring buoy or a “mooring structure” and that holds one or more riser lines from the well, to product piping ducts on the vessel. Since the turret mooring system should allow some rotation between the vessel and the buoy, the swivel is likewise adapted to provide rotation between the incoming fluid line and the product piping.
In particular for deep-sea applications there is a need for swivels that can withstand design pressures well over 500 atm (50.6 MPa) for incoming fluid while at the same time, a high flow of the fluid should be transferred.
Typically, a prior art swivel comprises a fixed (or static) annular element that is coupled to an incoming fluid line on the turret, and a rotating annular element that is coupled to the outgoing product piping on the vessel. The fixed and rotating annular elements each have a shell shape that form a closed toroidal volume between them in which a radial interface (a basically cylindrical surface area) is present between the rotating element as outer shell and the fixed element as inner shell. The radial interface allows the rotating element to rotate with respect to the fixed element, but in the same time is detrimental to swivel shell design by adding further surfaces exposed to the fluid pressure, calling thus for thicker shell wall.
In the swivel according to the prior art the outer shell of the swivel (i.e., the rotating annular element) will have the property of expanding under the pressure in the flow path between the inner and outer shell parts. Counteracting this expansion and associated stress imposes heavy wall thickness, and complex face seal design. This strategy is not indefinitely sustainable when the swivel diameter increases, since the radial force on the radial interface increases with increasing diameter. Especially for high pressure applications this will lead to unmanageable wall thicknesses and excessive weight of parts of the swivel. These issues have an adverse effect on the design and size of the swivel construction, but also on maintenance and repair, and of course have an important impact on the cost and on manufacturing limitations.
A further issue relates to the operation mode of prior art swivels which involves avoiding rubbing contact between the fixed and rotating parts. The load path across the various swivel parts is often a long loop passing by a deported roller bearing and which is sealed by seals that can only operate with a very small extrusion gap having very little variation. Again, a proper functioning of this known swivel can be obtained only via massive swivel parts.
It is an object of the present invention to overcome these disadvantages from the prior art.